While solar is a far less polluting energy source than coal or natural gas, many panel makers are nevertheless grappling with a hazardous waste problem.

To dispose of the material, the companies must transport it by truck or rail far from their own plants to waste facilities hundreds and, in some cases, thousands of miles away.The fossil fuels used to transport that waste, experts say, is not typically considered in calculating solar's carbon footprint, giving scientists and consumers who use the measurement to gauge a product's impact on global warming the impression that solar is cleaner than it is.

Nowhere is the waste issue more evident than in California, where landmark regulations approved in the 1970s require industrial plants like solar panel makers to report the amount of hazardous materials they produce, and where they send it.

The state records show the 17 companies, which had 44 manufacturing facilities in California, produced 46.5 million pounds of sludge and contaminated water from 2007 through the first half of 2011. Roughly 97 percent of it was taken to hazardous waste facilities throughout the state, but more than 1.4 million pounds were transported to nine other states: Arkansas, Minnesota, Nebraska, Rhode Island, Nevada, Washington, Utah, New Mexico and Arizona.Several solar energy experts said they have not calculated the industry's total waste and were surprised at what the records showed.

According to scientists who conduct so-called "life cycle analysis" for solar, the transport of waste is not currently being factored into the carbon footprint score, which measures the amount of greenhouse gases produced when making a product.Life cycle analysts add up all the global warming pollution that goes into making a certain product — from the mining needed for components to the exhaust from diesel trucks used to transport waste and materials. Not factoring the hazardous waste transport into solar's carbon footprint is an obvious oversight, analysts said.